Scenario: Enthusiastic response from the target audience (literary young adults who hate English class) to a series of posts on my blog has given me confidence that it’s a good non-fiction book concept. But there’s a dilemma: I can’t stop posting each new chapter draft on my blog, even though I want to sell the writing in a book.
But is this a problem, or an opportunity? Cory Doctorow’s Little Brother was given away online to promote the book. Nine Inch Nails does the same with music: give, then expect profit anyway.
But how would agents today feel, receiving a non-fiction book proposal that tells them that the first 10,000 words (so far) are already online with great reviews? And would it mollify them at all to be assured the online content is all rough draft, to undergo revision for the book?
Shorter version: Should I stop posting my installments if I want to sell them as a book?

One important point before I get to the meat of your question: both Cory Doctorow and Nine Inch Nails have a certain level of readership/fanbase that I’m just going to guess is slightly more than yours. They have the name recognition to give stuff away for free and still get people to pay for it.
That being said, I would have to recommend you stop putting additional chapters of your book up on the web. Consider that most non-fiction books are sold on proposal, not full manuscripts, and you may get a better sense of how much you’re already giving away. Yes, you want to lure people to your website, and yes, you want to have original material up there that can grow your readership and lead to the sort of name recognition that other bloggers have been able to convert into a book deal. But most of those blog-to-book deals have not been a strict repeat of material that’s already been posted. Lolcats didn’t just have published in book form the same cute pictures of cats that are already on the website. You don’t want to either.
You want to make sure that the material in your book is original, and is MORE than what someone could get just by going to your (free) website. And having rough drafts online and promising to polish the text for publication only means that website isn’t hosting your best work — is that what you want?
If I were you, I’d consider if you could take your already written work in a different direction for submission and possible publication, or see about revamping your website to remove what you want to eventually have published, and put up material that supports your book proposal, but isn’t the same thing.
Hope that helps!